Add Smith Normal Form, Jacobian group, and Picard group

[Copilot]

Implements the three follow-up items from the issue: a Smith Normal Form (SNF) routine, the Jacobian/critical group, the Picard group, and an SNF-based path through linear_equivalence.

Changes

• CFSmithNormalForm.py — smith_normal_form(A) -> (D, U, V) with U @ A @ V == D, U/V unimodular. Pure-integer arithmetic via NumPy object dtype (no float / no new deps). Plus elementary_divisors(A) helper.
• CFPicardJacobian.py — JacobianGroup(graph) and PicardGroup(graph), both built from the SNF of the graph Laplacian.
  • JacobianGroup: order (= number of spanning trees), invariant_factors, structure(), contains(divisor).
  • PicardGroup: free_rank, torsion_order, invariant_factors, structure(), equivalent(d1, d2).
  • free_rank reflects connected-component count; JacobianGroup.order raises on disconnected graphs.
• algo.linear_equivalence — new method kwarg: "ewd" (default, unchanged) or "smith", the latter dispatching to PicardGroup.equivalent for a purely algebraic check. Docstring notes that callers running many checks on one graph should reuse a PicardGroup to avoid repeated SNF computation.
• Exports added in chipfiring/__init__.py.
• Tests (tests/test_picard_jacobian.py, 31 cases) cover SNF invariants (U A V = D, unimodularity, divisibility chain), known group structures (K₃ → Z/3Z, K₄ → (Z/4Z)², trees → trivial, C₄ → Z/4Z), contains/equivalent membership, disconnected-graph handling, and EWD-vs-SNF agreement on K₄.

Example

from chipfiring import CFGraph, CFDivisor, JacobianGroup, PicardGroup, linear_equivalence

g = CFGraph({"a", "b", "c"}, [("a", "b", 1), ("b", "c", 1), ("a", "c", 1)])

JacobianGroup(g)            # JacobianGroup(order=3, structure='Z/3Z')
PicardGroup(g).structure()  # 'Z (+) Z/3Z'

d1 = CFDivisor(g, [("a", 3), ("b", 1), ("c", 0)])
d2 = CFDivisor(g, [("a", 1), ("b", 2), ("c", 1)])  # d1 with vertex a fired
linear_equivalence(d1, d2, method="smith")  # True